Nitrogen oxides, generally referred to as “NOx”, are harmful for the environment. For some time NOx-sensors are in common use with diesel engine vehicles as part of legal requirements regarding NOx emissions. The NOx-sensors used in high power diesel applications in heavy freight trucks are derived from NOx-sensors for the mainstream passenger car market. As a result the lifetime of the current NOx-sensors is much shorter than the designed lifetime of high powered freight vehicle diesel engines. Typically the current NOx-sensors have a lifespan of 200,000 to 300,000 road kilometers, whereas heavy trucks with diesel engines are designed to last between at least 1,200,000 to 1,600,000 road kilometers. This requires the NOx-sensors in heavy trucks to be checked and replaced on a regular basis. To properly check the correct functioning of an NOx-sensor it is currently necessary to remove these sensors from the vehicle's exhaust system and to have these tested by specialist testing services having the necessary costly apparatus to do so. Moreover the removal of these delicate NOx-sensors from an exhaust system also requires sealing arrangements to be renewed upon reinstallation. The risk therefor exists that the sensor is rendered defective by the test itself. As a result of more recent legislation vehicles equipped with NOx-sensors now also are required to have an on-board warning system to signal to the driver a possibly defective NOx-sensor. Such warning systems initiate a workshop visit to enable any malfunctioning of NOx-sensors to be excluded or to be corrected. Since adequate means for diagnosing NOx-sensors is unavailable in most workshops, more often than not expensive NOx-sensors are being replaced that turn out not to be defective, because the NOx-sensor warning was caused by another defect. This phenomenon has caused unnecessary high costs for truck owners, and also led to unnecessary warranty costs for truck manufacturers.